What is the difference between "Pose" and "metarig"
Every object in Blender has a transform/object-block and a data block refered to object and object data.


  Object
  Holds information about the position, rotation and size of a particular element.  
  
  Object Data
  Holds everything else.




There are various object data types. An object's object data can be swapped to another and multiple objects can share the same data. A null object owns no object data.

Object-data of the type armature is similiar. Your object-data is named "metarig". You can edit it by entering edit mode. When we enter edit mode for a mesh object (like a cube), the outliner stays the same. However, when we enter edit mode for an armature the outliner provides more information. When in edit mode, the armature object-data expand into a hierarchy. This feature is very helpful for editing the armature type object-data, since the hierarchical structure is important. (Meshes or curves don't display any information in the outliner, but that's ok, since the 3D View is a sufficient representation.)

Unlike all other object types, object with armature object-data have a third mode: pose mode. The bones created in edit mode are accessible in pose mode as well, but no for editing, just for posing. The pose bones also store transformation properties, but these are relativ to the rest pose. The restpose is deduced from the bone setup in edit mode and can only be changed in edit mode.

Next to the armature object-data, an armature object always will have pose-data.

The Pose datablock stores the bones pose. You will notice that when we enter edit mode the posebones disappear, because we can't edit the pose in edit mode, only the armature's object-data.



Are posebones and editbones the same?

In terms of describing a hierarchy, they look very similiar. But editbones (stored in the object-data "metarig") contain the rest pose of the armature and posebones (stored in the pose-data "Pose") store the pose of the bones. The posebones are also those that get keyframed. Edit bones can't be keyframed since the rest pose is unique.

Obviously an armature without a rest pose (the basis for all transformations and deformations) or a pose (the actual transformations and deformations) would not work, which is why you need both and cannot delete either.